<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style>
			* {
				box-sizing: border-box;
			}
			.parent {
				border: 5px solid #000;
			}
			.son1 {
				border: 5px solid #f00;
				float: left;
				width: 33.333%;
				margin-right: 1px;
			}
			.son2 {
				border: 5px solid #0f0;
				float: left;
				width: 33.333%;
			}
			.son3 {
				border: 5px solid #00f;
				float: left;
				width: 33.333%;
			}
		</style>
	</head>
	<body>
		<div class="parent">
			<div class="son1">1</div>
			<div class="son2">2</div>
			<div class="son3">3</div>
		</div>
	</body>
</html>
